Frictional resistance of ceramic and stainless steel orthodontic brackets Don H. Pratten, DMD, MS," Kris Popli, BS, b Nicholas Germane, DMD, c and John C. Gunsolley, DDS, MS d Richmond, Va.

Frictional resistance of orthodontic appliances is recognized by most clinicians to be detrimental to tooth movement. The purpose of this study was to compare planar, static frictional forces among stainless steel and ceramic brackets. Both nitinol and stainless steel rectangular arch wires were passed freely through the slots of a pair of brackets from each type. Tests were carried out in air and in artificial saliva. A 300-gin load was suspended from the arch wire to simulate the normal force, and an incremental horizontal force was applied until movement of the arch wire was initiated. Under all conditions, the stainless steel brackets had lower coefficients of friction than the ceramic brackets. The stainless steel wire generated less friction than nitinol, and friction increased in the presence of artificial saliva in comparison with air alone. These results show that, under experimental conditions, ceramic brackets, nitinol arch wires, and saliva all increase static frictional resistance. (AMJ ORTHOD DENTOFACORTHOP 1990;98:398-403.)

T h e static and kinetic frictional forces generated between brackets and arch wires during sliding mechanics should be minimized to allow optimal tooth movement) "2 Although numerous investigators have studied the sliding mechanics of metal and plastic appliances, 38 no studies have compared the frictional forces generated by ceramic and stainless steel brackets. The use of translucent ceramic brackets, an esthetic alternative to stainless steel on anterior teeth, has grown rapidly in the past several years. However, the sliding mechanics of these new materials may present the practitioner with a less efficient method of moving teeth. The purpose of this study was to compare differences in magnitude of planar, static, frictional force generated by ceramic brackets and stainless steel brackets, in combination with nitinol and stainless steel arch wires, in air and in artificial saliva. MATERIALS AND METHODS

Three brands of 0.018-inch orthodontic brackets were used in this study: (1) ceramic A (Allure, GAC International, Central Islip, N.Y.), (2) ceramic B (Transcend, Unitek Corp., Monrovia, Calif.), and (3) stainFrom the School of Dentistry, Medical College of Virginia, Virginia Commonwealth University. 'Assistant Professor, Department of Restorative Dentistry. bDental student. CAssistant Professor, Department of Orthodontics. aAssistant Professor, Departments of Periodontics and Biostatistics. 811116586

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less steel (Unitek). Two pairs of brackets of each material were used in conjunction with each of two types of rectangular arch wire, size 0.017 inch × 0.022 inch: (1) nitinol (Unitek) and (2) stainless steel (Unitek). Each pair, in combination with an arch wire, was tested five times in both air and artificial saliva media (Orex, Young Dental, St. Louis, Mo.). Frictional forces were measured by an instrument (Fig. 1) in which straight pieces of arch wire were allowed to pass freely through the slots of a pair of brackets securely mounted I cm apart on an acrylic plate. A 300 gm weight, representing a constant normal force, was suspended from the arch wire between the brackets. A horizontal force was applied to the arch wire by means of a string-pulley system connected to a water container. Drops of water from a pipette were allowed to fill the container at a constant rate until sliding of the arch wire could be initiated. The total weight of the container and water, measured in grams, was designated as the frictional force. The effects of different types of wire or bracket on the mean values of frictional force, both in the absence and in the presence of artificial saliva, were analyzed by means of a three-way analysis of variance. RESULTS

The results of the frictional resistance comparisons are shown in Figs. 2, 3, and 4. Under all conditions, the stainless steel brackets generated lower frictional

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forces than the ceramic brackets. Analysis of variance showed a statistically significant difference between ceramic and stainless steel brackets (p < 0.001), but not between the two ceramic bracket bands (p < 0.82). The stainless steel arch wire generated less friction than the nitinol arch wire (p < 0.001). The artificial saliva medium increased static frictional forces in comparison with air (p < 0.001) for all combinations of bracket and arch wire types. The greatest increase (35%) in static friction caused by artificial saliva occurred with the use of stainless steel brackets and stainless steel arch wire (Fig. 4). DISCUSSION

The results of this study show that ceramic brackets provide significantly greater frictional resistance than

stainless steel brackets when they are used in combination with either stainless steel or nitinol arch wires. Because the use of ceramic brackets will require increased sliding force or decreased ligature force, practitioners will want to consider this difference when selecting brackets for tooth movement that involves sliding mechanics. Friction is known to be determined in large part by surface roughness. 9 The significantly lower frictional resistance provided by stainless steel brackets is most likely a result of their lower surface roughness, which is clearly visible when comparing scanning electron micrographs (Figs. 5 and 6). Nitinol arch wires generated significantly greater frictional resistance than stainless steel. This finding supports data reported by Garner et al. 5 and Stannard

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Am. J. Orthod. Dentofac. Orthop. November 1990

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et al. 7 This difference in friction, however, is not easily explained by the scanning electron micrographs in Figs. 7 and 8, which do not show appreciable differences in surface texture. The presence of an oxide layer or intrinsic lubrication may influence the friction of these materials more than surface roughness. A previous study that compared nitinol and stainless steel arch

wires reported lower friction for the nitinol only when wire-bracket angulation was >5°. H Since no angulation was used in the present experiment, the two studies could not be compared. Saliva substitute was found to increase static friction for all combinations tested. This finding contradicts the general perception of saliva as a lubricant for arch wires

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Fig. 5. Scanning electron micrograph of stainless steel bracket. (Original magnification x 1000.)

Fig. 6. Scanning electron micrograph of ceramic bracket. (Original magnification x 1000.)

and brackets. A study by Baker et al. t° of the effect of saliva on friction showed a statistically significant reduction in frictional force of 15% to 19%. A study by Andreasen and Quevedo, 4 however, found that saliva played an insignificant role in lubricating the surfaces of the wire or the bracket slot. The explanation for the

discrepancies may lie in the significance of the loading forces used between the arch wire and the brackets. At low loads saliva acts as a lubricant, but at high loads saliva may increase friction if it is forced out from the contacts between the brackets and the arch wire. In the latter situation, saliva may produce shear resistance to

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°

Fig. 7. Scanning electron micrograph of stainless steel arch wire. (Original magnification x 1000.)

Fig. 8. Scanning electron micrograph of nitinol arch wire. (Original magnification x 1000.)

sliding forces. The normal force in this study was kept constant at 300 gm. This value is of the same order as that used in other studies and is typical of ligature forces used during translational tooth movement? '4 All friction measurements in this study were ob-

tained from materials in planar apposition--i.e., flat surfaces of rectangular wires were drawn across flat surfaces of bracket slots. Such simple geometry permits an uncomplicated examination of basic differences in frictional resistance of materials. Since the clinical sit-

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uation involves binding between brackets and wire, additional measurements at various wire angulations would be interesting.

7.

REFERENCES

8.

!. Nikolai RJ. Bioengineering analysis of orthodontic mechanics. Philadelphia: Lea & Fcbiger, 1985:53-6. 2. Proffit WR. Contemporary orthodontics. St. Louis: CV Mosby, 1986:236. 3. Frank CA, Nikolai ILl. A comparative study of frictional resistances between orthodontic bracket and arch wire. AM J ORTItOD 1980;78:593-609. 4. Andreasen GF, Quevedo FR. Evaluation of frictional forces in the 0.022 x 0.028 edgewise bracket in vitro. J Biomech 1970;3:151-60. 5. Garner LD, Allai WW, Moore BK. A comparison of frictional forces during simulated canine retraction of a continuous edgewise arch wire. AM J ORTIXOI9DENTOFACORTttOP 1986;90:199203. 6. Feeney FJ. The effects of bracket width, wire dimension, and sliding force magnitude on bracket-wire friction. [Master's the-

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sis]. Farmington, Connecticut: University of Connecticut, 1988. 55 p. Stannard JG, Gau JM, Hanna MA. Comparative friction of orthodontic wires under dry and wet conditions. AM J OR'nZOD 1986;89:485-91. Riley JL, Garrett SG, Moon PC, Frictional forces of Iigated plastic and metal edgewise brackets. J Dent Res 1979;58:98. Bowden FP. The friction and lubrication of solids, vol 1. Oxford: Clarendon Press, 1964:I-5. Baker KL, Nieberg LG, Weimer AD, Hanna M. Frictional changes in force values caused by saliva substitution. AM J ORTttOD DENTOFACORTItOP 1987;91:316-20. Peterson L, Spenser R, Andreasen G. A comparison of friction resistance for nitinol and stainless steel wire in edgewise brackets. Quintessence Int 1982;13:563-71.

Reprint requests to: Dr. Don Pratten MCV/VCU Box 566 MCV Station Richmond, VA 23298-0566

Frictional resistance of ceramic and stainless steel orthodontic brackets.

Frictional resistance of orthodontic appliances is recognized by most clinicians to be detrimental to tooth movement. The purpose of this study was to...
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